1. Field of the Invention
This invention generally relates to optical/electrical devices and, more particularly, to an optical/electrical device with a pre-attached lens to expand the effective aperture and optical beam width.
2. Description of the Related Art
As noted in Wikipedia, the vertical-cavity surface-emitting laser (VCSEL) is a type of semiconductor laser diode with laser beam emissions perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers which emit from surfaces formed by cleaving the individual chip out of a wafer. The VCSEL has many potential advantages over the edge-emitting lasers. Its design allows chips or dies to be manufactured and tested on a single wafer. Large arrays of devices can be created exploiting methods such as flip-chip optical interconnects and optical neural network applications to become possible. In the telecommunications industry, the VCSEL's uniform, single mode beam profile is desirable for coupling into optical fibers. However, with these advantages come a number of problems particularly in the fabrication and operation at high powers.
There are many designs of VCSELs, however, they all have certain aspects in common. The cavity length of VCSELs is very short typically 1-3 wavelengths of the emitted light. As a result, in a single pass of the cavity, a photon has a small chance of a triggering a stimulated emission event at low carrier densities. Therefore, VCSELs require highly reflective mirrors to be efficient. In edge-emitting lasers, the reflectivity of the facets is about 30%. For VCSELs, the reflectivity required for low threshold currents is greater than 99.9%. Such a high reflectivity cannot be achieved with the use of metallic mirrors. VCSELs make use Distributed Bragg Reflectors (DBRs). These are formed by laying down alternating layers of semiconductor or dielectric materials with a difference in refractive index. At the dispersion minima for optical fibers, semiconductor materials used for DBRs have a small difference in refractive index, therefore, many periods are required. Since the DBR layers also carry the current in the device, more layers increase the resistance of the device. Therefore, dissipation of heat and growth can become a problem.
FIG. 1 is a partial cross-sectional view of a metallic reflector VCSEL (prior art). Today, most VCSEL devices employ quantum wells within the cavity. By depositing a thin layer of semiconductor with a slightly smaller band gap, one cannot only define a region for recombination to occur, one can control the optical properties of the device. Discrete energy levels are formed in the conduction and valence bands. The power obtained from a single quantum well is low. Multiple quantum wells may be grown within the cavity to increase power obtained.
The reduced cavity length in VCSELs and the addition of quantum wells significantly reduces the probability of stimulated emission in a single pass of the cavity. The light within the cavity must be reflected back into the cavity many more times than with a Fabry Perot laser. The average time the photons spend within the cavity is known as the photon lifetime. The reflectivity of the mirrors must be very high to increase the photon lifetime and thus the time of interaction with the excited electron states.
VCSELs for wavelengths from 650 nm to 1300 nm are typically based on gallium arsenide (GaAs) wafers with DBRs formed from GaAs and aluminum gallium arsenide (AlxGa(1-x)As). Longer wavelength devices, from 1300 nm to 2000 nm, have been demonstrated with at least the active region made of indium phosphide.
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. The conventional solar cell used to generate electric solar power is a large area photodiode. Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays), or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode use a PIN junction rather than the typical p-n junction to increase the speed of response. A photodiode is designed to operate in reverse bias.
When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a positively charged electron hole. This mechanism is also known as the photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimized to enhance the sensitivity of the device.
One problem with the use is laser diode devices made using convention IC fabrication procedures is the “quality” of the optical beam created. Ideally, the beams should be confined to a collimated path with a narrow beam diameter. In practice, the beam may be dispersed in a pattern defined by a cone angle. To aid in beam collimation, a lens may be used in conjunction with a laser diode. Likewise, to compensate for the misalignment of a collimated beam, or to aid in the collection of a dispersed beam, a lens may be used in conjunction with a photodiode to gather a greater intensity of light.
Unfortunately, although photodiodes and VCSELs can be fabricated on wafers using convention IC fabrication processes, it has proved difficult to fabricate these devices with pre-attached lenses. As a result, the lens must be integrated with the VCSEL or photodiode as a separate component at a higher level, after the wafer has been diced and the individual photodiode or VCSEL devices are assembled into an end product device. Integration at this level typically requires that the placement of the lens be adjusted with respect to the photodiodes or VCSELs. This low scalability alignment procedure may involve the use of precision mechanical elements—slowing production and adding to costs.
It would be advantageous if lens could be integrated with laser diodes and photodiodes at the wafer level.